Playing HoG level one, I noticed at least 4 different permanent Spirit Workers lacking some portion of health. I healed them, but they were found damaged again. It also seems they lack the Worker Regen ability. :P
Yeah technically they can't be attacked, but they can still be damaged by sources such as lava.
Confirmed with lead designer. It is intended that spirit workers should not be able to be attacked. However they should not be able to take damage.
I tested Lava and it can confirm that lava does not damage spirit workers. They also cannot be damaged by AOE however I believe that it is possible for a Spirit Worker to be directly hit by a projectile intended for another target.
This is being changed for the next patch.
Since they are invisible, it's probably lava :)